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rectangular input
Posted Jul 20, 2009, 3:58 p.m. EDT 1 Reply
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I would like to add a concentration only for a predetermined time, i.e. rectangular input instead of step input. How should I type this in the COMSOL as a expression? Will it take anything like this?
C0*( u(t)-u(t-2))
C0*( u(t)-u(t-2))
1 Reply Last Post Jul 20, 2009, 6:25 p.m. EDT
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Posted:
2 decades ago
Hi
why u(t) ? what do you mean by that ?
u for me has a default dimension of [m] in COMSOL, check carefully your dimensions.
But you could use a C0*(t>0.5)*(t<1), this would make a step function between 0.5 and 1.
Now, do not forget that this step function does not have any defined derivative at the discontinuities, so your solver will probably not manage to pass the steps !
The are the Heaviside functions (fldc1hs, flc2hs, flsmhs, flsmsign ...) to make smooter steps with contiuous derivatives, they work mostly better, see the doc .
By the there was another discussion about steps functions about a month ago
Good luck
Ivar
why u(t) ? what do you mean by that ?
u for me has a default dimension of [m] in COMSOL, check carefully your dimensions.
But you could use a C0*(t>0.5)*(t<1), this would make a step function between 0.5 and 1.
Now, do not forget that this step function does not have any defined derivative at the discontinuities, so your solver will probably not manage to pass the steps !
The are the Heaviside functions (fldc1hs, flc2hs, flsmhs, flsmsign ...) to make smooter steps with contiuous derivatives, they work mostly better, see the doc .
By the there was another discussion about steps functions about a month ago
Good luck
Ivar